Office 2013 click-to-run does not honor deselecting 'Make Outlook the default program' checkbox

Question

We use an email client other than Microsoft Outlook (let's say Thunderbird)
Thunderbird is set as the default mail client for the machine (HKLM\Software\Clients\Mail\(default) set appropriately)
Occasionally, we launch Outlook (but absolutely do not want it as the default mail client for the machine).  About 30 seconds after launching Outlook, we see that the Windows default mail client changes to Microsoft Outlook.The 'Make Outlook the default
program for E-mail, Contacts, and Calendar' checkbox in File->Options is disabled.
Further investigation with ProcMon indicates that a SYSTEM service called integrator.exe starts up about 30 seconds after we launch Outlook.  This service changes the HKLM\Software\Clients\Mail\(default) registry key to be Microsoft Outlook.
We have tested with multiple non-Outlook email clients, and the behavior is always the same.
This only happens on click-to-run installs of Office 2013.  Regular installs of Office 2013 do not have the problem.  Older versions of Outlook do not have the problem.
What can we do to get Outlook to honor the 'Make Outlook the default program' setting?

Answer 1

Kapaal -
Thanks for your reply.  Your blog post would certainly work if this was just a user-mode app that was making the change, but that's not the case with Office 2013.
The integrator.exe process that is making the change is part of Microsoft's Click-to-Run system.  It runs as a service with local system access, and thus would have full access to the key.  I might be able to lock it down by creating a second system
admin account and playing games with it - but that will make it very difficult to actually change the value when I want it do be changed, and I really shouldn't have to be doing workarounds to prevent Office itself from doing this.
- Kevin